Tilton.] 504 [-'^P"l •'• 
between the basin rocks and tlie granite are found, the contacts 
lie near the granite highlands bordering the basin, and not at a 
distance inward toward the basin. It seems desirable, then, 
that where the exact position is not determined by the natm-e of 
the outcrops, the line should be drawn near the granite rather 
than along the nearest basin rock, or half way between the out- 
crops of the granite and basin rock. If this criterion be fol- 
loAved, the line must be drawn southward, including the Farm 
Pond outcrops of conglomerate as a part of the general basin 
area, rather than across from ledge to ledge north of Farm Pond, 
leaving these southern conglomerates in a small isolated basin. 
The outcrop just south of the bridge over Charles River east 
of Farm Pond is not very conspicuous, but as the pressure- 
planes conform to the general direction of such planes, and the 
slightly raised extent of ground beneath the outcrops seems to 
indicate an underlpng ledge, this locality is included in the 
granite area. 
Along the river near by to South Xatick, the granite forms an 
almost continuous irregular slope on the eastern side of the river ; 
but immediately east of Farm Pond are a few outcrops near the 
road. The northern one of those east of the road is an arkose 
rock ; the one a little to the south, a felsite. These rocks are so 
similar in their composition and appearance on weathering that 
no marked line of contact is visible ; but because of the difference 
in their microscopic character, the line between the basin and the 
granite is judged to lie across this ledge. 
Turning now from these evidences that the Boston Basin 
extends from South Natick to south of Farm Pond, let us con- 
sider briefly the conditions that exist east of South Xatick in the 
large valley north of Dover. 
As in the preceding description the line separating the basin 
area from the surrounding granite has been di-awn along the out- 
crops of granite, so, from east of South Xatick to near Charles 
River Village, the course of the line must be decided on the same 
general principle. 
On the west side of the valley are the granite heights of Pegan 
Hill, from which two extensions run eastward into the valley. 
These extensions present a low rounded api)earance as if deter- 
